Because explosive compositions are sensitive to heat, pressure, friction and shock, the pumping of such compositions should be done in a manner to eliminate the presence or creation of hazardous conditions caused by exceeding safe limits for these variables. For example, in a progressive cavity pump, internal friction exists between the rotor and stator, and this friction continually generates heat during the pumping operation. The amount of heat generated normally is small enough that it simply can be transferred to and dissipated within the composition being pumped, without heating the composition to an undesirable temperature. When little or no flow exists in the pump because of a closed or restricted outlet or lack of composition flow to the inlet, however, the frictional heat can accumulate in the rotor and stator themselves, which after a period of time, can become hot enough and can transfer enough heat to the composition to cause its ignition.
Some approaches have been used or suggested for addressing this explosive composition pumping problem. Safety shutdown systems are utilized that monitor electronically the temperature and pressure parameters at various locations in or proximate to the pump, so that if the conditions exceed set maximum limits the systems will automatically shut down the pump's operation. This or similar monitoring observations have been done manually, with a pump operator present to observe the live operations of the pump. Both of these approaches are potentially deficient in that they are susceptible to mechanical (or electronic) or human error, and if error occurs, the pump may continue to operate and create the overheating phenomenon described above.
European Patent Application (EPA) publication No. 0 255 336 discloses another approach for eliminating or reducing the possibility of overheating occurring in a progressive cavity pump. Disclosed is a connection between the drive shaft of the pump and the rotor that comprises a heat-sensitive, breakaway bond of a heat-fusable metal alloy, the metal alloy connection being meltable upon the generation of heat within the pump cavity so as to disconnect the mechanical linkage between the drive shaft and the rotor. This approach has the disadvantages of being relatively expensive to build and to replace (following an event of overheating and disconnection) when compared to the meltable stator of the present invention. Further, internal friction and heat generation will continue to occur between the drive shaft and the metal alloy. A number of earlier U.S. patents are cited and described in this EPA publication that address different approaches for sensing the operating conditions of rotary pumps.
In spite of these prior approaches, a need exists for a reliable, simple, fail-safe and economic means for preventing the overheating of a progressive cavity pump that is pumping explosive compositions or other heat-sensitive materials.
The present invention satisfies this need by providing a meltable, elastomeric stator that has a melting temperature at a predetermined level above the normal or desired operating temperature of the pump but below the thermal reaction temperature of the explosive composition or heat-sensitive material being pumped. This melting of the stator causes the accumulation of heat to cease so that the temperature within the pump decreases and becomes essentially constant at a temperature below the thermal reaction temperature of the explosive composition or other material. In this way, the temperature of the pump simply cannot increase above the undesired thermal reaction temperature, because the melting stator no longer provides any resistance and thus friction to the rotating rotor. The melting occurs naturally and simply as the result of the temperature increase in the pump, and thus is not dependent upon external controls or monitors. It has an additional advantage over the approach disclosed in EPA 0 255 336 in that the stator melts in direct response to the operating temperature within the pump cavity and thus within the explosive composition being pumped, rather than first requiring the conduction of heat through the rotor to the connection between the rotor and the drive shaft. This conduction process takes time and could result in the composition reaching an undesired temperature before the connection breaks away. Another advantage of the meltable stator is that it easily and inexpensively can be retrofitted to existing pumps that are used for pumping heat-sensitive materials.
Thus the present invention provides an important safety enhancement to progressive cavity pumps that are used to pump explosive compositions and other heat-sensitive materials.